1. Field of the Invention
The present invention relates to an original reading apparatus for use in a copying machine, an image signal transmitting equipment, or the like.
2. Description of the Prior Art
A typical example of an apparatus of the type mentioned above is an apparatus in which an original is illuminated by a light source such as a fluorescent lamp, and the light reflected from the original is condensed by a condenser and is projected onto a reading element such as a CCD. Such an apparatus, however, encounters a problem in that the light source, such as a fluorescent lamp, inevitably has some variation in the quantity of light emitted therefrom between the longitudinal end portions and the central portion of the lamp. Further, a beam of light which has passed through the condenser lens has a light distribution such that the quantity of light at the center thereof is large while that at both ends is small, this characteristic being known as the Cosine.sup.4 Law. As a result, the distribution of light projected onto the reading element becomes non-uniform. This phenomenon is known as "shading" and may affect resulting image signals.
In order to avoid any distortion of image signals due to shading, it has been common practice to adopt shading correction. For this purpose, a known original reading apparatus is provided with a white plate disposed at a position at which a moving original is read, and shading correction is conducted by illuminating the white plate with a light source, detecting the light reflected from the white plate by means of the reading element, and, by using this detection signal as a reference signal, electrically correcting the signal output from the reading element which corresponds to the image on the original.
With such a known apparatus, the white plate is disposed at a position corresponding to the reverse side of an original, such as a sheet of paper having an image, as it moves through the original reading position, and the reference signal for shading correction is obtained by illuminating the white plate before the apparatus starts to read the original, and detecting the light reflected from the white plate by means of the reading element. In such a known apparatus, if it is necessary to read a stationary original which is, for instance, provided by one or more pages of a book placed motionless on an original table, shading correction is performed before the start of reading of the stationary original, in a manner similar to that as described as above. That is, the light source is fixed at the original reading position, the white plate is illuminated with the light source, and a reference signal for shading correction is obtained. Thereafter, the light source is moved to a position enabling reading of the stationary original.
This known apparatus, however, fails to form any reference signal for shading correction in some cases. In particular, when it is necessary to continuously read a plurality of original sheets, when the quantity of light emitted from the light source fluctuates within a relatively short period because of factors such as low temperature, or when the light source lamp remains turned on for a long period during the reading of a lengthy original, it is impossible to obtain a reference signal during the reading of the original by using the white plate disposed on the reverse side of the original. In these cases, therefore, the known apparatus may form image signals distorted by shading, thus providing images of inferior quality.
An original reading apparatus may be arranged as described below when it is necessary to read a stationary original, such as a page of a book, placed motionless on the original table. The light source is first turned on while it is fixed at a stop position, and, after the quantity of light emitted from the light source has become stable, the white plate is illuminated by the light source to form a reference signal for shading correction. Thereafter, the light source is moved to read the stationary original.
Such an arrangement is, however, disadvantageous because the movement of the light source can be started only after the reference signal has been formed, thus necessitating a long period for the entire reading operation.
In order to cope with the first mentioned problem, U.S. Pat. No. 4,614,976 proposes providing, in addition to a white plate disposed on the reverse side of the original, a white plate which is movable to a desired position over the side of the original facing the light source. With this arrangement, the additional white plate can be moved, when necessary, to an original illuminating position to enable the formation of a reference signal for shading correction.
In accordance with this proposal, however, a mechanism has to be provided for moving the movable white plate, thus causing an increase in production cost and in the size of the entire apparatus.